It is known in the styrene-butadiene polymerization art that the use of a randomizing modifier together with an anionic initiator system generally increases the vinyl content of the 1,3-diene contributed units in the copolymers. The use of randomizing modifiers also presents problems in the polymerization process as the modifier or its degradation products must be recovered after polymerization has been completed.
It is an object of the present invention to provide anionic initiator systems for styrene-butadiene type copolymers which randomize the styrene type monomers in the copolymers without specifically utilizing randomizing modifiers.
It is a further object of the present invention to provide anionic initiator systems which produce styrene-butadiene type copolymers having low vinyl contents in the 1,3-diene monomer contributed units.